192 A. Hadjidakis 
apply to hydrofoil craft with fully submerged foils, lacking a definite position of equilibrium 
relative to the water surface. Their stability depends only on a human or electric brain, 
controlling the lift developed by the different foils. The behavior in a seaway of this latter 
category depends merely on the degree of intelligence of its governor. 
Furthermore, only the most unfavorable conditions have been considered. ‘Going against 
the waves, the vertical accelerations were found to be critical; when going with the waves, 
this was the case with the maximum negative pitch angle. 
Both critical values depend on design characteristics of the different hydrofoil systems, 
as there are: the lift reserve of the forward foils, the natural frequencies for pitch and 
heave, the damping ratio, etc. These design characteristics, being independent of size, 
will not be discussed in this paper. 
The many purposely introduced simplifications, of course, create deviations. Thus it 
is necessary to apply the results only to two or more craft of the same hydrofoil system, 
differing essentially in a scale factor only, for it is only in that case that the deviations are 
in the same sense for all units, so that they will largely compensate each other. 
THE SEAWAY 
The waves are assumed to be of sinusoidal shape, where the waveheight H is equal to 
o times the wavelength A, and o indicates the rate of steepness of the waves. The craft’s 
speed V forms an angle y with the vector of the speed of wave propagation c. Then the 
forced frequency or excitation frequency of the forces, trying to disturb the craft from its 
equilibrium position, is 
ce V. : Xr 
w = 27 [2- Fees where C = aa (1) 
The amplitude of the static pitch angle on a given wave pattern can be determined as 
follows: The maximum static pitch angle y, is attained in the position shown in Fig. 1. 
Its value is 
ee a Jen (ed (2) 
which can never exceed zo. 
Fig. 1. Maximum static pitch angle 
